The present invention relates to a device for selecting needles in a circular knitting machine, in particular for stockings.
Countless devices are known for selecting needles in knitting machines. In particular, a device is known, described in the U.S. patent application Ser. No. 06/916,904 filed on Oct. 7, 1986 by the same applicant. Such a device consists of a plurality of superimposed selection levers which are arranged facing the needle-bearing cylinder in the region of the selectors or sub-needles. The selection levers are individually pivoted at an intermediate portion thereof to a supporting framework and can oscillate in a plane substantially parallel to the extension of the plurality of levers, that is, parallel to the direction of sliding of the selectors. By virtue of the oscillation of the selection levers it is possible to have their end facing towards the selectors move from a first position, in which this end is at an intermediate level between the heels of the selectors so as to not interfere therewith, to a second position wherein said end is at the level of the heels of the selectors so as to interfere therewith, so as to activate or not activate specific selectors.
For the oscillation of the selection levers, actuating levers are used which are oscillable in a plane substantially parallel to the plane of oscillation of the selection levers and which have one of their ends acting on the end of the selection levers facing in the opposite direction with respect to the selectors.
In order to reduce the dimensions of the actuating levers in a direction parallel to the axis of the needlebearing cylinder of the machine, and to simplify assembly, the end of the selection levers which is coupled to the related actuating lever is in the shape of a flat blade and the related end of the actuating lever is bifurcated to engage said blade on both sides.
For the oscillation of the actuating levers, electromagnets and a permanent magnet, counterposed with respect to the actuating lever, are used. The electromagnet is connected to an electronic machine control element which attracts it according to a preset processing program, while the permanent magnet is used for the return of the actuating lever when the electromagnet is deactivated.
Such a type of device, though it achieves a satisfactory reduction in dimensions and quicker assembly, as well as an excellent operation at medium speeds, has proved to be susceptible to improvements.
With the ever-increasing speeds required in knitting machines, it is in fact desirable to have an actuating lever return speed which is substantially equal to the speed imparted by the electromagnet when it is activated. However, the replacement of the permanent magnet with an electromagnet, keeping the same position, can lead to an interference in magnetic fields if, as required in the case of high operating speeds, the value of the voltage is relatively high.
Furthermore, a presence of a permanent magnet can cause problems during the assembly and the operation of the device, since metallic particles can be attracted between the actuating lever and the permanent magnet, thus reducing the reaction speed of the magnet.
Another problem is due to the fact that when the electromagnet is activated it is necessary to maintain a relatively high voltage in order to overcome the continuous attraction force of the permanent magnet. Since a high voltage causes the overheating of the electromagnet, a very precise setting of the voltage is required in order to avoid damage. Voltage stabilizers are used for this reason.